This invention relates to a rare gas cold cathode discharge tube for an image input device used for inputting images into computers.
An image input unit used for inputting images such as photographs and the like into computers may be classified by its reading system. The first type is a camera type image input unit which functions to read at 20 milliseconds or less using photoelectric transducer elements in a two-dimensional array. This type of device is mainly used for reading images which change with time. However, since the optical path length for imaging must be set during construction, a large space is required for the unit. Additionally, the picture image must be irradiated entirely by light of a uniform brightness. Thus, it is difficult to obtain an accurate density value of the picture image. A highly precise manufacturing technique, which increases the cost, is also necessary since the photoelectric transducer elements are arranged two-dimensionally.
The second type of image input device is a drum scanner having an imaging system for reading one point and photoelectric transducer elements. To read the picture image, while rotating a picture image on a drum, the photoelectric transducer elements are shifted axially to the direction of rotation. The reading resolution is controlled by selecting a reading resolution arbitrarily by controlling the speed of the drum and the rate of movement of the photoelectric transducers. Thus a relatively high resolution may be attained. However, since the resolution depends on the mechanical precision of the component parts, the cost of the device may be high. Additionally, the device is unavoidably large.
The final type of image input device is an image scanner having photoelectric transducer elements such as a CCD (charge coupled device) and the like in a one-dimensional array. The image is read by shifting the photoelectric transducer elements relative to a picture image in a direction vertical to the array.
The image scanner type device has advantages over the camera type device and the drum scanner type device. The image scanner type device has a higher read rate than the drum type device and is smaller than both the camera type and drum type devices. Thus an image scanner having photoelectric transducer elements in a one-dimensional array is relatively inexpensive, compact and high in resolution.
An LED array, a fluorescent lamp, a linear halogen lamp or the like are employed as the lighting apparatus used in the image scanner to irradiate the picture image in the direction in which the photoelectric transducer elements are arrayed. However, a tone representation capacity is required in an image input unit. Unless the density value of the picture image is quantized into, for example, 8 to 256 gradations for loading, a picture image such a photograph or the like with a fine change in intermediate density cannot be accurately loaded. Thus the picture image must be irradiated uniformly by a constant brightness, and, consequently, a lighting apparatus providing a stable amount of light is necessary.
A rare gas cold cathode discharge tube charged with xenon or neon has a feature that the amount of light produced is almost constant regardless of temperature of the working environment in comparison to a general fluorescent lamp tube using mercury as shown in FIG. 16. Accordingly, when a conventional fluorescent lamp tube charged with mercury is used, it must be warmed up by a heating apparatus such as an electric heater or the like. Thus a time of between about 1 and 2 minutes or so is required before actuation. The rare gas cold cathode discharge tube, however, is ready for use as soon as the power is turned on. The electrode is small in shape, and the tube is miniaturized in overall size as, for example, between about 1 and 6 mm in diameter, since the electrode is not heated. The power consumption of the rare gas cold cathode discharge tube is also low, for example, between about 4 and 10 watts, and the luminous color is arbitrarily selected by choosing the fluorescent material applied inside the tube. Thus a rare gas cold cathode discharge tube is appropriate not only for a facsimile but also as a light source of an image scanner for reading color picture images.
If a picture image read on a color image scanner is printed directly by a color printer, the picture image obtained will be dark, different in hue from the original picture image and inferior in saturation since the reflective spectral characteristics of the existing color inks are not ideal. Thus color correction is necessary to correct color imbalance due to the spectral characteristics of the inks used on the printer.
In order to perform the color correction process, density values for the three primary colors, green, red and blue, are necessary for each picture element corresponding to the reading resolution. The volume of data required is extremely large and therefore a device for performing color correction is expensive, and the time required for calculating color correction is long. Thus, brightly colored printed matte is not achievable using general computers.
Color correction is performed quickly and cheaply by a line sequential reading system. The conventional system, a page sequential system, required the entire color picture to be read three times in green, red and blue. In the line sequential system, however, data of the three primary colors, green, red and blue, is loaded at every reading. Thus the full picture image is read in one scan. In the line sequential reading system, the volume of data necessary for color correction work may be minimized to one several thousandths of the data is a page sequential system (in the case of A4 size paper). Using a semiconductor RAM capable of writing and reading as a storage device for color correction, and also by providing an integrated circuit for color correction in the image scanner, color correction can be carried out during the reading, and the color correction data may thereafter be sent to a host computer.
As described above, compared with a general fluorescent lamp tube charged with mercury, the rare gas cold cathode discharge tube provides a more stable amount of light with environmental temperature change and is more compact. However, it has defects as when used as a light source for reading picture images. That is, the amount of light produced during intermittent lighting, which is necessary for line sequential reading, is not stable. Once it is lit, a rare gas cold cathode discharge tube can be used as continuously maintained lighting for several seconds or longer.
In a rare gas cold cathode discharge tube, the charged gas pressure is high, between about 50 and 200 mmHg, while in a fluorescent lamp tube charged with mercury the charged gas pressure is several tens mmHg. Thus a straight bright line called a positive column is observed along the discharge tube at the time of lighting. To stably locate the positive column at a specified portion of the rare gas cold cathode discharge tube, an auxiliary electrode is provided along a wall of the rare gas cold cathode discharge tube, thereby emitting light.
In the prior art, when repeating the intermittent lighting at a period of several milliseconds or so, the positive column is not stabilized and drawn toward the auxiliary electrode, the amount of light of the rare gas cold cathode discharge tube is not constant, and the brightness of the read image changes. Specifically, while the positive column exists at all times, the light emitting position fluctuates within the discharge tube to approach or go away from the desired picture image. Thus the quantity of light for irradiating the picture image fluctuates 1 to 10 percent.
In high performance image reader for reading a fine density picture image at gradations of 32 to 256, even this small fluctuation in the quantity of light may exert an influence on the reproduced picture, and a stripe is produced even though the picture image read had a uniform density.
Accordingly, it is desirable to provide an improved image input device having a light source which produces a stable amount of light during intermittent lighting.